When a plurality of substrates are joined with solder, it is necessary to consider the reliability and workability during and after the joining of the substrates. The following will describe a junction structure of substrates according to the prior art.
FIG. 5 shows a first prior art example as a junction structure of substrates according to the prior art. Reference numeral 21 denotes a first substrate, reference numeral 22 denotes a second substrate, reference numeral 23 denotes the metal foil portions of the first substrate 21, and reference numeral 24 denotes the metal foil portions of the second substrate 22. The first substrate 21 and the second substrate 22 are arranged in parallel, the metal foil portions 24 of the second substrate 22 are overlapped on the metal foil portions 23 of the first substrate 21, and solder joints are formed thereon.
A second prior art example as a junction structure of substrates according to the prior art is disclosed in Japanese Patent Laid-Open No. 9-320092. FIG. 6 shows the junction structure. In FIG. 6, reference numeral 31 denotes a first flexible substrate, reference numeral 32 denotes a second flexible substrate, reference numeral 33 denotes a holding member (optical base) for holding the second flexible substrate 32, reference numeral 34 denotes a positioning protrusion for positioning the holding member 33, and reference numeral 35 denotes solder applied to the copper foil portions of the second flexible substrate 32. The first flexible substrate 31 and the second flexible substrate 32 are caused to cross each other at substantially right angles and are joined with solder.
A third prior art example as a junction structure of substrates according to the prior art is disclosed in Japanese Patent Laid-Open No. 1-248687. FIG. 7 shows the junction structure. In FIG. 7, reference numeral 41 denotes a first flexible printed circuit board having comb-teeth portions 42a formed on the ends of metal-foil exposed portions 42 provided on the end of the first flexible printed circuit board 41. Reference numeral 43 denotes a second flexible printed circuit board having holes 44 formed next to metal-foil exposed portions 45 of the second flexible printed circuit board 43 such that the comb-teeth portions 42a can be inserted into the holes 44. During joining, the comb-tooth portions 42a are inserted into the holes 44 and then soldering is performed. Further, at least one electrical component 47 is mounted so as to be hidden under the first flexible printed circuit board 41 and the angle of insertion into the holes 44 is regulated according to the height of the electrical component 47.